Method and apparatus for inserting programmable latency between address and data information in a memory tester

ABSTRACT

A memory tester has a feature including a method and an apparatus, to programmably insert a latency between address and data channels. Address information is stored in a FIFO memory during a first program instruction cycle. After a desired number of program instruction cycles, the address information is retrieved during a second program instruction cycle. The retrieved address information is used to address a location in a tester memory for storage of data information received from a DUT. In this way, the data information is correlated to a latent address according to DUT specifications.

BACKGROUND

Electronics devices and capabilities have grown extremely common in daily life. Along with personal computers in the home, many individuals carry more than one productivity tool for various and sundry purposes. Most personal productivity electronic devices include some form of non-volatile memory. Cell phones utilize non-volatile memory in order to store and retain user programmed phone numbers and configurations when the power is turned off. PCMCIA cards utilize non-volatile memory to store and retain information even when the card is removed from its slot in the computer. Many other common electronic devices also benefit from the long-term storage capability of non-volatile memory in un-powered assemblies.

Non-volatile memory manufacturers that sell to the electronic equipment manufacturers require testers to exercise and verify the proper operation of the memories that they produce. Due to the volume of non-volatile memories that are manufactured and sold at consistently low prices, it is very important to minimize the time it takes to test a single part. Purchasers of non-volatile memories require memory manufacturers to provide high shipment yields because of the cost savings associated with the practice of incorporating the memory devices into more expensive assemblies with minimal or no testing. Accordingly, the memory testing process must be sufficiently efficient to identify a large percentage of non-conforming parts and preferably all non-conforming parts in a single test process.

As non-volatile memories become larger, denser and more complex, the testers must be able to handle the increased size and complexity without significantly increasing the time it takes to test them. As memories evolve and improve, the tester must be able to easily accommodate the changes made to the device. Another issue specific to testing non-volatile memories is that repeated writes to cells of the memories can degrade the overall lifetime performance of the part. Non-volatile memory manufacturers have responded to many of the testing issues by building special test modes into the memory devices. These test modes are not used at all by the purchaser of the memory, but may be accessed by the manufacturer to test all or significant portions of the memories in as little time as possible and as efficiently as possible. Some non-volatile memories are also capable of being repaired during the test process. The tester, therefore, should be able to identify: a need for repair; a location of the repair; the type of repair needed; and, must then be able to perform the appropriate repair. Such a repair process requires a tester that is able to detect and isolate a specific nonconforming portion of the memory. In order to take full advantage of the special test modes as well as the repair functions, it is beneficial for a tester to be able to execute a test program that supports conditional branching based upon an expected response from the device.

From a conceptual perspective, the process of testing memories is an algorithmic process. As an example, typical tests include sequentially incrementing or decrementing memory addresses while writing 0's and 1's into the memory cells. It is customary to refer to a collection of 1's and 0's being written or read during a memory cycle as a “vector”, while the term “pattern” refers to a sequence of vectors. It is conventional for tests to include writing patterns into the memory space such as checkerboards, walking 1's and butterfly patterns. A test developer can more easily and efficiently generate a program to create these patterns with the aid of algorithmic constructs. A test pattern that is algorithmically coherent is also easier to debug and use logical methods to isolate portions of the pattern that do not perform as expected. A test pattern that is generated algorithmically using instructions and commands that are repeated in programming loops consume less space in tester memory. Accordingly, it is desirable to have algorithmic test pattern generation capability in a memory tester.

Precise signal edge placement and detection is also a consideration in the effectiveness of a non-volatile tester. In order to capture parts that are generally conforming at a median while not conforming within the specified margins, a nonvolatile memory tester must be able to precisely place each signal edge relative in time to another signal edge. It is also important to be able to precisely measure at which point in time a signal edge is received. Accordingly, a non-volatile memory tester should have sufficient flexibility and control of the timing and placement of stimuli and responses from the Device Under Test (memory).

Memory testers are said to generate transmit vectors that are applied (stimulus) to the DUT, and receive vectors that are expected in return (response). The algorithmic logic that generates these vectors can generally do so without troubling itself about how a particular bit in a vector is to get to or from a particular signal pad in the DUT. At this level it is almost as if it were a certainty that adjacent bits in the vector would end up as physically adjacent signals on the DUT. Life should be so kind!

In reality, the correspondence between bits in a vector at the “conceptual level” and the actual signals in the DUT is apt to be rather arbitrary. If nothing were done to prevent it, it might be necessary to cross one or more probe wires as they descend from a periphery to make contact with the DUT. Such crossing is most undesirable, and it is convention to incorporate a mapping mechanism in the path of the transmit vector to rearrange the bit positions in the transmit vector before they are applied to the DUT, so that task of making physical contact is not burdened with crossings. Receive vectors are correspondingly applied to a reverse mapping mechanism before being considered. In this way the algorithmic vector generation and comparison mechanisms can be allowed to ignore this entire issue. As another example of what such mappers and reverse mappers can do, consider the case when a different instance of the same type of DUT is laid out on the same wafer, but with a rotation or some mirrored symmetry, in order to avoid wasting space on the wafer. These practices also have an effect on the correspondence between vector bit position and physical signal location, but which can be concealed by the appropriate mappings and reverse mappings. It will be appreciated that the mappings and reverse mappings needed for these situations are, once identified for a particular DUT, static, and need not change during the course of testing for that particular DUT. Many memories have latency specifications, which refer to the difference in a number of cycles between presentation of addressing information to the DUT, and reception of data from the DUT that logically correlates to the addressing information. Many memories have multiple latency specifications. All of the latency specifications must be tested for a complete test of a single kind of device. Prior semiconductor testers have required custom configuration of the tester or different test programs for each latency requirement. Custom configuration of the tester is undesirable in that it is expensive, prone to error, and difficult to support. Different test programs for each latency requirement is undesirable in that it increases the overhead and time to test a single part and also increases test development and maintenance effort. There is a need, therefore, for a tester that supports configurable testing of multiple latencies in a single test program.

SUMMARY

An apparatus for correlating data information latent from address information in a memory tester comprises a first in first out latency memory having address information available to it. A first signal is synchronized with a first instruction cycle in the tester and causes the latency memory to store the address information in a next available write location. A second signal is synchronized with a second instruction cycle in the tester and causes the latency memory to retrieve the address information from a next read location in the latency memory. A tester memory accepts the retrieved address information for accessing storage locations in said tester memory in order to correlate data information to said retrieved address information.

According to another aspect of the invention, a method for correlating data information latent from address information in a memory tester comprises the steps of storing the address information in a first instruction cycle, retrieving the address information in a second instruction cycle, and using the address information stored in the first instruction cycle to access a location in a tester memory for storing data information from the second instruction cycle.

Advantageously, a tester according to the teachings of the present invention offers additional flexibility and efficiency in developing test patterns compared to prior art testers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an extensively reconfigurable non-volatile memory tester constructed in accordance with the invention.

FIG. 2 is a simplified block diagram expansion of the DUT tester 6 of FIG. 1.

FIG. 3 is a block diagram of the address mapper 29 shown in FIG. 2.

FIG. 4 is a block diagram of the programmable latency portion 332 of the address mapper 29 shown in FIG. 3.

FIG. 5 is a listing of some sample code illustrating source code that uses latency program mnemonics.

FIG. 6 is a block diagram of the data multiplexer 35 shown in FIG. 2.

DETAILED DESCRIPTION

Refer now to FIG. 1, wherein is shown a simplified block diagram 1 of a Non-Volatile Memory Test System constructed in accordance with the principles of the invention. In particular, the system shown can simultaneously test, with as many as sixty-four test points each, up to thirty-six individual DUT's (Devices Under Test) at one time, with provisions for reconfiguration to allow elements of a collection of test resources to be bonded together to test DUT's having more than sixty-four test points. These test points may be locations on a portion of an integrated circuit wafer that has not yet been diced and packaged, or they might be the pins of a packaged part. The term “test point” refers to an electrical location where a signal may be applied (e.g., power supplies, clocks, data inputs) or where a signal can be measured (e.g., a data output). We shall follow the industry custom of referring to the test points as “channels”. The “collection of test resources to be bonded together” referred to above may be understood as being as many as thirty-six test sites, where each test site includes a Test Site Controller (4), a (sixty-four channel) DUT Tester (6) and a (sixty-four channel) collection of Pin Electronics (9) that makes actual electrical connection to a DUT (14). In the case where testing the DUT requires sixty-four or fewer channels, a single Test Site is sufficient to perform tests upon that DUT, and we say, for example, that the Test Site #1 (as it appears in FIG. 1) forms or operates as a “Single Site Test Station”. On the other hand, when some form of the aforementioned reconfiguration is in effect, two (or more) Test Sites are “bonded” together to function as one larger equivalent Test Site having one hundred and twenty-eight channels. Accordingly, and again in reference to an example shown in FIG. 1, we say that Test Sites #35 and #36 form a “two-Site Test Station”.

To briefly consider the opposite case, one should not assume that an entire Test Site is needed to test a single DUT, or that a single Test Site can test but a single DUT. Suppose that a wafer had two (probably, but not necessarily, adjacent) dies, the sum of whose test channel requirements were sixty-four channels or less. Both DUT's can be tested by a single Test Site. What makes this possible is the general purpose programmability of each Test Site. A test program executed by the Test Site may be written such that one part of the Test Site's resources is used to test one of the DUT's while another part is used to test the other DUT. After all, we would assume that if we had a third DUT that were the logical union of the first two, then we would be able to test that third DUT with a single Test Site, so we ought to be able to similarly test its “component DUT's”, as it were. The only difference is individually keeping track of whether the two “component DUT's” pass or fail, as opposed to a unified answer for the “third” DUT (that is, there is an issue concerning what portion of the “third” DUT failed). This “Single-Site Multi-Test Station” capability is largely conventional, and we mention it here for the sake of completeness, and to ward off potential confusion and misunderstanding when comparing it to the notion of bonding two or more Test Sites together.

Were it not for this notion of reconfiguration there would be no difference between a Test Site and a Test Station, and we would dispense with one of the terms. As it is, however, it will be readily appreciated that the number of Test Stations need not equal the number of Test Sites. In the past, the numbers could be different because Test Sites were split to create more Test Stations (DUT's not complex enough to consume an entire Test Site). Now, however, the difference may also be due to Test Sites having been bonded together to form multi-site Test Stations (DUT's too complex for a single Test Site).

To continue, then, a Test System Controller 2 is connected by a System Bus 3 to as many as thirty-six Test Site Controllers whose names end in the suffixes #1 through #36 (4 a-4 z). (It is true that subscripts a-z only go from one to twenty-six, and not to thirty-six. But this minor deception seems preferable over numerical subscripts on numerical reference characters, which would be potentially very confusing.) The Test System Controller 2 is a computer (e.g., a PC running NT) executing a suitable Test System Control Program pertaining to the task of testing non-volatile memories. The Test System Control Program represents the highest level of abstraction in a hierarchical division of labor (and of complexity) for accomplishing the desired testing. The Test System Controller determines which programs are being run by the different Test Sites, as well as overseeing a robotics system (not shown) that moves the test probes and DUT's as needed. Test System Controller 2 may function in ways that support the notion that some Test Sites are programmed to perform as single-site Test Stations, while others are bonded together to form multi-site Test Stations. Clearly, in such circumstances there are different parts being tested, and it is most desirable that different tests be used for the different parts. Likewise, there is no requirement that all single-site Test Stations be testing the same style of part, nor is there any such requirement for multi-site Test Stations. Accordingly, the Test System Controller 2 is programmed to issue the commands to accomplish the needed Test Site bonding and then to invoke the appropriate test programs for the various Test Stations in use. The Test System Controller 2 also receives information about results obtained from the tests, so that it may take the appropriate action for discarding the bad part and so that it may maintain logs for the various analyses that may be used to control, say, production processes in a factory setting.

The Test System itself is a fairly large and complex system, and it is common for it to use a robotics subsystem to load wafers onto a stage that then sequentially positions one or more future dies under probes connected to the Pin Electronics 9, whereupon those future dies (the wafer has not yet been diced) are tested. The Test System can also be used to test packaged parts that have been loaded onto a suitable carrier. There will be (as is explained below), at least one Test Site Controller associated with each Test Station in use, regardless of how many Test Sites are used to form that Test Station, or of how many Test Stations are on a Test Site. A Test Site Controller is an embedded system that may be an i960 processor from Intel with thirty-six to sixty-four MB of combined program and data memory running a proprietary operating system called VOS (VersaTest O/S), which was also used in earlier products for testing non-volatile memories (e.g., the Agilent V1300 or V3300). For the moment, we shall consider only the situation for single-site Test Stations. For the sake of a definite example, suppose that Test Site #1 is functioning as Test Station #1, and that it is to test the WHIZCO part no. 0013. The test regimen involves a hundred or so different types of tests (varying and monitoring voltage levels, pulse widths, edge positions, delays, as well as a large dose of simply storing and then retrieving selected patterns of information), and each type of test involves many millions of individual memory cycles for the DUT. At the highest level, the operators of the Test System instruct the Test System Controller 2 to use Test Station #1 to begin testing WHIZCO 0013's. In due course the Test System Controller 2 tells Test Site Controller #1 (4 a) (which is an embedded [computer] system) to run the associated test program, say, TEST_WHIZ_13. If that program is already available within Test Site Controller #1's environment, then it is simply executed. If not, then it is supplied by the Test System Controller 2.

Now, in principle, the program TEST_WHIZ_13 could be entirely self-contained. But if it were, then it would almost certainly be rather large, and it may be difficult for the processor of the embedded system within the Test Site Controller 4 a to run fast enough to produce the tests at the desired speed, or even at a rate that is uniform from one DUT memory cycle to the next. Accordingly, low level subroutine type activities that generate sequences of address and associated data that is to be written or is expected from a read operation, are generated as needed by a programmable algorithmic mechanism located in the DUT Tester 6, but that operates in synchrony with the program being executed by the embedded system in the Test Site Controller 4. Think of this as exporting certain low level subroutine-like activity and the task of initiating DUT memory cycles out to a mechanism (the DUT Tester) that is closer to the hardware environment of the DUT 14. Generally speaking, then, whenever the Test System Controller 2 equips a Test Site Controller with a test program it also supplies the associated DUT Tester with appropriate low level implementation routines (perhaps specific to the memory being tested) needed to accomplish the overall activity described or needed by the programming for the Test Site Controller. The low level implementation routines are termed “patterns”, and they are generally named (just as functions and variables in high level programming languages have names).

Each Test Site Controller #n (4) is coupled to its associated DUT Tester #n (6) by a Site Test Bus #n (5). The Test Site Controller uses the Site Test Bus 5 to both control the operation of the DUT Tester and receive therefrom information about test outcomes. The DUT Tester is capable of generating at high speed the various DUT memory cycles that are involved in the test regimen, and it decides if the results of a Read memory cycle are as expected. In essence, it responds to commands or operation codes (“named patterns”) sent from the Test Site Controller by initiating corresponding useful sequences of Read and Write DUT memory cycles (i.e., it executes the corresponding patterns). Conceptually, the output of the DUT Tester 6 is stimulus information that is to be applied to the DUT, and it also accepts response information therefrom. This stimulus/response information 7 a passes between the DUT Tester 6 a and a Pin Electronics #1 assembly 9 a. The Pin Electronics assembly 9 a supports up to sixty-four probes that can be applied to the DUT 14.

The above-mentioned stimulus information is just a sequence of parallel bit patterns (i.e., a sequence of “transmit vectors” and expected “receive vectors”) expressed according to the voltage levels of some family of logic devices used in the DUT Tester. There is a configurable mapping between bit positions within a stimulus/response and the probes on the die, and this mapping is understood by the DUT Tester 6. The individual bits are correct as to their timing and edge placement, but in addition to the mapping they may also need voltage level shifting before they, can be applied to the DUT. Likewise, a response that originates in the DUT subsequent to a stimulus may need buffering and (reverse) level shifting before it can be considered suitable for being fed back to the DUT Tester. These level shifting tasks are the province of the Pin Electronics 9 a. The Pin Electronics configuration needed for testing a WHIZCO 0013 likely will not work for testing a part from the ACME Co., and perhaps not even with another WHIZ Co. part. So, it will be appreciated that the Pin Electronics assembly needs to be configurable also; such configurability is the function of the PE Config lines 8 a.

The above concludes a brief architectural overview of how a single Test Site is structured for testing a DUT. We turn now to issues that arise when there are many Test Sites with which to operate. As a preliminary, we shall describe a preferred embodiment for constructing a Test System having multiple Test Sites. In many respects, some of the information we are about to describe are matters of choice based on market studies of customer preference and cost benefit analyses. Be that as it may, to build one of these things one has to make definite choices, and once that is done there are particular consequences that are visible throughout the entire system. It is felt that it is useful to describe, at least in a general way, the larger outlines of the hardware properties of the Test System. Even though some of these properties are contingent, knowledge of them will nevertheless assist in an appreciation of various examples used to illustrate the invention.

To begin, then, consider four rather large card cages. Each card cage has, besides power supplies and water cooling (fans can be a source of contamination in a clean room environment), a motherboard, a front plane and a back plane. Into each card cage can be placed up to nine assemblies. Each assembly includes a Test Site Controller, DUT Tester and Pin Electronics. We shall be describing the general outlines of how Test Site Controllers are bonded together, which will involve some busses used to create daisy chains.

A brief digression concerning the term “daisy chain” is perhaps in order. Consider system elements A, B, C and D. Suppose that they are to be daisy chained together in that order. We could say that there is an information or control path that leaves A and goes into B, that B can selectively pass on traffic that then leaves B and goes into C, and that C can selectively pass on traffic that then goes into D. These same kinds of arrangements can exist for traffic in the other direction, too. Daisy chains are often used to create priority schemes; we shall use them to create master/slave relationships between various the Test Site Controllers. We shall denote these daisy chained style communication arrangements with the suffix noun “DSY”, instead of “BUS”. Thus, we might refer to a Command /Data DSY instead of a Command/Data Bus. Now, the notion that information “enters B and is selectively passed on” may suggest that traffic is replicated onto a separate set of conductors before being passed on. It could be that way, but for performance reasons it is more like a regular bus having addressable entities. By means of a programmable address mapping arrangement and the ability to put portions of downstream Test Site Controllers “to sleep,” the single bus can be made to logically appear (i.e., to function) as a plurality of daisy chains. Finally, it will be appreciated that the daisy chains are high performance pathways for command and control information, and that if they were not, then we could not expect a master/slave combination (multi-site Test Station) to operate as fast as a single Test Site does. For the benefit of daisy chain performance, the various DSY do not leave their respective card cages. The effect of this decision is to place some limits on which Test Sites (and thus also how many) can be bonded together. In principle, there is no fundamental need for this limitation, nor is there a genuine lack of technical practicality involved (it could be done); it is simply felt that, since there are already nine Test Sites in a card cage, extending the DSY's adds significant cost for relatively little additional benefit.

To resume our discussion of FIG. 1, then, consider the various Test Site Controllers 4 a-4 z that can populate the four card cages, each with nine Test Site Controllers. Let's denote them as 4 a-4 f, 4 g-4 m, 4 n-4 t and 4 u-4 z. (Never minding, as explained earlier, that these are nominally only twenty-six subscripts—the reader is invited to imagine that there are another ten subscript symbols in there, someplace.) A CMD/DAT DSY 17 a (Command & Data Daisy Chain) interconnects the Test Site Controller 4 a-4 f that are in one card cage, while a different CMD/DAT DSY 17 b interconnects the Test Site Controllers 4 g-4 m in another card cage. The same arrangement exists for the remaining card cages, and Test Site Controllers 4 n-4 t and 4 u-4 z, respectively. We have earlier said that the DSY do not leave the card cages, in that “tail end” of a bus that actually forms the DSY does not leave a card cage and become the head of the next segment in another card cage. Instead, the System Bus 3 from the Test System Controller 2 goes to all Test Site Controllers, and each is capable of becoming a Master at the head of a DSY segment that does not leave the card cage.

The CMD/DAT DSY 17 a-d that we have been discussing exist between the various Test Site Controllers 4 a-4 z. There is a similar arrangement for the SYNC/ERR DSY 18 a-18 d and the DUT Testers 6 a-6 z. The synchronization and error information conveyed by the SYNC/ERR DSY 18 allows DUT Testers to function in unison. These two daisy chains (17 and 18) carry slightly different types of information, but each exists as part of the same general mechanism for bonding one or more Test Sites together into a Test Station.

We turn now to a discussion of FIG. 2, which is a simplified block diagram expansion of the DUT tester 6 of FIG. 1, of which there may be as many as thirty-six. It is sufficient at present to describe only one instance thereof. A glance at FIG. 2 will show that it is a fairly well populated with stuff; especially so for a “simplified” block diagram. Some of what is in the DUT Tester 6 and represented in the block diagram is functionally quite complicated, and is not available in “off the shelf” form. It is appropriate here to make two points. First, the primary purpose of including FIG. 2 is to describe the basic properties of an important operational environment within the overall Non-Volatile Memory Test System 1. The invention(s) that are fully described in connection with FIG. 3 and subsequent figures will either be expansions of mechanisms set out in the following description of FIG. 2, or they will be new mechanisms whose motivational premise is found in FIG. 2. Either way, as this is written it is not known exactly which of these is before the reader. The goal at present is to provide a simplified yet informative starting point for numerous different Detailed Descriptions of various Preferred Embodiments, so that each of those can be as concise as is appropriate (as opposed to one “jumbo” Specification that discloses everything about each different invention). The second point is that the expanded or extended material, while in general overall agreement with FIG. 2, may contain information that does not “match-up” exactly with the simplified version. This does not mean there has been an error, or that things are fatally inconsistent; it arises because it is sometimes difficult or impossible to simplify something such that it is the exact image in miniature. The situation is rather like maps. A standard size road map of Colorado will show that when going east on I-70 you can go north on I-25 at Denver. It looks like a left turn. And while it did used to be an actual left turn, it isn't one now, and a detailed map of that intersection will show a sequence of component turns and intervening road sections. But no one would say that the standard size road map is wrong; it is correct for its level of abstraction. Similarly, and despite its fairly busy appearance, FIG. 2 is indeed a simplification operating at a medium level of abstraction, but some seeming left turns are not simple left turns at all.

As is shown in FIG. 1, the major input to the DUT Tester 6 is an instance of the Test Site Bus 5, which originates from a Test Site Controller 4 that is associated with the instance of the DUT Tester 6 that is of interest. The Test Site Bus 5 is coupled to a Micro-Controller Sequencer 19, which may be likened to a special purpose microprocessor. It fetches instructions from a program stored in a program memory, which may be either internal to the Micro-Controller Sequencer 6 (PGM SRAM 20) or external thereto (EXT. DRAM 21). Although these two memories appear to be addressed by what is essentially a logically common address 63 that serves as a program counter (or, instruction fetch address), and either can be a source of programming to be executed, note that: (1) Only one of the memories performs instruction fetch memory cycles during any period of time; and (2) In fact they are addressed by electrically different signals. The SRAM is fast and allows genuine random access, but consumes valuable space within the Micro-Sequence Controller 19 (which is a large IC), so its size is limited. The external DRAM can be provided in adjustable amounts of considerable quantity but is fast only when accessed in sequential chunks involving linear execution and no branching. Programming in the SRAM 20 most often that which is intensely algorithmic, while the EXT. DRAM 21 is best suited for material not readily generated by algorithmic processes, such as initialization routines and random or irregular data.

The instruction word executed by the Micro-Controller Sequencer 19 is fairly wide: two hundred and eight bits. It consists of thirteen sixteen-bit fields. These fields often represent fetched instruction information for mechanisms that are outside the Micro-Controller Sequencer proper. Such fields are dedicated to their associated mechanisms. One set of ALU INSTRUCTIONS 22 are applied to a collection of eight sixteen-bit ALU's 24, while others are disbursed to various other mechanisms distributed throughout the DUT Tester. This latter situation is represented by the lines and legend “VARIOUS CONTROL VALUES & INSTRUCTIONS” 42.

The eight sixteen-bit ALU's (24) each have a conventional repertoire of arithmetic instructions built around associated sixteen-bit result registers (each ALU has several other registers, too). Three of these result registers and their associated ALU's are for generating X, Y and Z address components 27 that are variously combined into a complete address to supply to the DUT. Two more of the eight ALU/registers (DH & DL) are provided to assist in the algorithmic creation of thirty-two bit data patterns 28 that are divided between a most significant portion (DH) and a least significant portion (DL). A final three ALU/registers (A, B, C) are used as counters and contribute to the production of various PROGRAM CONTROL FLAGS 25 that assist with program control and branching on completion of some programmatically specified number of iterations or other numerical condition. These PROGRAM CONTROL FLAGS 25 are sent back to the Micro-Controller Sequencer 19, where they affect the value of the instruction fetch address in ways familiar to those who understand about microprocessors. There are also various OTHER FLAGS 55 that also can be used to effect program branching. These originate with various ones of the other mechanisms within the DUT Tester 6 that are controlled by the different fields of the fetched instruction word. One specific additional flag is expressly shown as a separate item: VEC_FIFO_FULL 26. In another drawing having somewhat less detail it might be lumped in along with the OTHER FLAGS 55. We have separated it out to assist in explaining one aspect of the operation of the Micro-Controller Sequencer 19.

What VEC_FIFO_FULL does is to (temporarily) halt further program execution by the Micro-Controller Sequencer 19. There are many stages of pipeline between the instructions fetched by the Micro-Controller Sequencer 19 and the mechanism that finally hands test vectors off to be applied to the DUT. In addition, part of the baggage that accompanies a vector as it moves toward being applied to the DUT is information concerning the rate of eventual vector application, or, each vector's duration. Thus, the rate of vector application to the DUT need not be constant, and in particular, a group of vectors may take longer to apply than they did to generate. The Micro-Controller Sequencer simply executes programming at its maximum rate. But clearly, on average, the rate of “vector consumption,” as it were, must equal the rate of “vector production,” lest the pipeline need to be elastic nearly without limit. There is a Vector FIFO 45 at the output of the Address Mapper 29 discussed below, and it serves as an elastic capacity in the pipeline. The signal VEC_FIFO_FULL is used to prevent overrunning the limited number of stages in the pipeline, by causing a temporary cessation in the production of new vectors at the head end of the pipe.

To continue, the (three times sixteen equals forty-eight bits of) X, Y and Z address components 27 are applied to an Address Mapper 29, whose output is a selected-in-advance nearly arbitrary rearrangement of the address values in the ordered forty-eight bit address space. As a point of departure for appreciating this, suppose for a moment that the Address Mapper 29 were a memory that fully populated a forty-eight bit address space, and that it held a forty-eight bit value at each address. (Temporarily never mind that such a memory would—today anyway—be size of a large refrigerator.) Given such a memory, a look-up table could be implemented that could map any applied address into another, arbitrarily selected, forty-eight bit value which could then be used as a replacement address. The reason that such address mapping is desirable is that the X, Y and Z address components generally have useful meaning in the context of a particular DUT's internal architecture, which is most likely not implemented with one big linear decoder. The notions of rows, columns and layers, block or pages may be very useful to the Test Engineer, and failures that occur in locations that are physically close together may involve corresponding closeness in their X, Y and Z addresses. Such patterns in the test results can be valuable in appreciating what is wrong and in trying to fix it, whether at a design level or at a production level of reprogramming a part to shunt a defective section's operation with that of a spare section. Two issues arise from such thinking. The first is paring the forty-eight bits down to the actual number of bits (say, thirty-two, or perhaps sixteen) to be applied to the DUT. We shall shortly briefly mention how the paring down is done, and it is largely a matter of taking this many bits from X, that many from Y and the rest from Z. But not entirely, and this is the second issue, because certain addresses might lie within circuitry that is a left-for-right (or left-for-right and top-for bottom) mirror image of another section of circuitry. This has the effect of rearranging what the bits mean, as far as what sequential address values are in physical order within that circuitry. This chip layout property may occur many times, and it may well be the case that how one group of bits for, say, Y, are interpreted, may depend upon the accompanying value of some other, say, Z bits. The address mapper 29 is provided to allow the raw X, Y and Z addresses to be “repackaged,” as it were, to reflect this sort of thing for the benefit of those who would test memories having such internal architectural arrangements. As to how it's actually done, the Address Mapper 29 is constructed of a fairly large number of interconnected multiplexers. It cannot implement the completely arbitrary look-up table behavior of a fully populated memory decode scheme as was temporarily assumed above for purposes of explanation. It can however, rearrange sub-fields of the X, Y and Z address components as needed, particularly since there is yet another mechanism that will do the paring down from forty-eight bits to the actual number needed. The Address Mapper 29 also contains three sixteen bit (address) look-up tables that allow it to perform limited arbitrary mapping within local ranges.

The mapped address output 30 of the Address Mapper 29 is applied as an address to an Aux RAM 31 and to an Error Catch RAM 32, which, while having separate functions, may nevertheless be implemented selectable partitions in one larger overall RAM. The mapped address output 30 is also applied as one input to an Addr. Bit Select circuit 37, which is described below.

Consider the Aux RAM 31. Its function is to retain data patterns 33 and addresses 34 that can be applied to the DUT. These are logically separate outputs from the Aux RAM 31, since they are treated somewhat differently and used in different places. (The AUX RAM 31 is not a dual “port memory”, but is preferably of several banks whose outputs are applied to MUX's.) In keeping with this, it may be that Stored Data 33 is kept in one bank or range of addresses of the Aux RAM 31, while Stored Addresses 34 are kept in another. Also, we have not shown an explicit mechanism for writing to the Aux RAM 31. That is accomplished by an addressed bus operation initiated by a Test Site Controller 4 at the behest of the program it is executing. (There is an “under the floorboards,” as it were, “utility services” bus called the “Ring Bus” [not shown—as it would clutter the drawing immensely] that goes to just about everything in FIG. 2.)

The Error Catch RAM 32 is addressed by the same address that is applied to the Aux RAM 31, and it either stores or retrieves information about errors, which operations are performed in conjunction with a Post Decode Circuit, to be discussed later. As with the paths 33 and 34 from the Aux RAM 31, paths 61 (into the Error Catch RAM) and 62 (from the Error Catch RAM) are preferably MUX'ed outputs from a multi-bank memory (the Error Catch RAM 32), in accordance with configuration information distributed by the Ring Bus (not shown).

Note that the Data MUX 35 has as inputs the STORED DATA output 33 from the Aux RAM 31 as well as data 28 from the registers DH and DL in the collection 24 of ALU's. The Data MUX 35 selects which of these inputs (28, 32) to present as its output 38, which is then applied as one of two vector components to a Transmit Vector Mapper/Serializer/Receive Vector Compare Data Circuit 40 (the other component is the output 39 of the Addr. Bit Select circuit 37). Bit Select circuit 37). Data MUX 35 performs this selection in accordance with values 36 stored in PGM SRAM 20.

Circuit 40 can perform three functions: assemble vector components (38, 39) into an ordered logical representation an entire vector that is to be applied (transmitted) to the DUT; apply an arbitrary dynamic correspondence (mapping) between the ordered bits of the logical representation of the transmit vector and the actual physical channel number of the Pin Electronics (i.e., which probe tip) will contact the DUT on behalf of that signal (i.e., that bit in the vector); and, cooperate with the compiler in the division of an entire logical vector into pieces to be applied separately and in order (serialization) for DUT's that admit of such a thing. Which of these functions is performed is determined by control signals from an SRAM 41, which is also addressed in accordance with a field in the two hundred and eight bit instruction fetched by the Micro-Controller Sequencer 19. The output of Circuit 40 is an up to sixty-four bit vector 44 that is applied to a Vector FIFO 45, which when full generates the signal VEC_FIFO_FULL 26, whose meaning and use was discussed above. The vector at the top of the Vector FIFO 45 is removed therefrom upon receipt of a signal VEC_FIFO_UNLOAD 47 that originates at a Period Generator 49 (to be discussed shortly). Such removed vectors (46) are applied to a Timing/Formatting & Comparison circuit 52 that is connected to the DUT via the associated instance of Pin Electronics 9. That is, each instance of Pin Electronics 9 receives Transmitted & Received Vectors 7 and Pin Electronics configuration information 8 from its associated Timing/Formatting & Comparison circuit 52.

The Timing/Formatting & Comparison circuit 52 has an Internal SRAM 54 addressed by the same Instruction Address (“A” in the small circle) as is the Program SRAM 20 of the Micro-Controller Sequencer 19. (An External DRAM 53 may be used in place of the Internal SRAM 54.) The Internal SRAM 54 (or external DRAM 53) assists in the production of Drive and Comparison cycles. Drive cycles apply a transmit vector to the DUT. Comparison cycles receive a vector presented by the DUT and examine it to determine if it matches previously supplied comparison data. Both Drive and Comparison cycles are adjustable as to their duration, whether and when a load is applied, and when data is latched or strobed. The comparison produces a sixty-four bit value 56 that is applied to a Receive Vector Reverse Mapper/Deserializer 57, whose function may be considered to be the logic 40. (The operation of circuit 57 is controlled by an SRAM 58 that corresponds to the control of circuit 40 by SRAM 41.) In turn, the output 59 of circuit 57 is applied to the Post Decode circuit 60. At present, it is sufficient to say that the Post Decode circuit 60 can inspect via programmatic criteria both incoming error information 59 and (previously) stored error information 60 (stored in Error Catch RAM) to produce condensed and more readily interpretable error information which may then by stored back into the Error Catch RAM 32 via path 61. An example would be to create a count of how many times there was an error within a particular range of addresses, which information may be useful in deciding when to attempt to engage in on-chip repair by enabling substitute circuits.

We turn now to the Period Generator 49 and its associated Timing SRAM 51. These respond to an eight bit signal T_SEL 43 that, for each two hundred and eight bit instruction fetched by the Micro-Controller Sequencer 19, determines a duration for the associated operation of the Timing/Formatting & Comparison circuit 52. T_SELT 43 is member of the Various Control Values & Instructions 42 that are represented by the different fields within the fetched instruction. As an eight bit value it can represent or encode two hundred and fifty-six different things. In this case those “things” are twenty-eight bit values stored in the Timing SRAM 51 and that are addressed by T_SEL. Each addressed twenty-eight bit value (23) specifies a desired duration with a 19.5 picosecond resolution. The sequence of accessed twenty-eight bit duration values (23) is stored in a Period FIFO 50 so that the individual members of that sequence will be retrieved and applied in synchronism with the retrieval of their intended corresponding vector, which is stored in the Vector FIFO 45.

A coarse timing value field in the oldest entry in the FIFO 50 conveys duration information with a resolution of 5 nsec, and produces therefrom a signal VEC_FIFO_UNLOAD 47 that transfers the next transmit vector from the Vector FIFO 45 to the Timing/Formatting & Comparison circuit 52. A companion signal TIMING REMAINDER 48 is also applied to circuit 52. It is there that the ultimate resolution to 19.5 picoseconds is accomplished.

One known feature of a memory tester is a mapping feature, which, among other things, assigns a logical pin allocation to a physical pin allocation. A test developer need not adhere to the most efficient and electrically pragmatic pin assignments between the DUT 14 and the tester channels and can logically address DUT pins in terms of their function. As an example, a 16-bit x-address bus on a DUT 14 may be disposed in four(4) different physical locations on the DUT die. As a result of the disjointed placement, the channels in the test site may be some arbitrary combination of channels. Rather than requiring the test developer to follow the physical block address mapping, the scramble feature helps perform the task of mapping the physical channels to the logical channels in the test program. Furthermore, a single test pattern may be useful for more than one kind of device. Different devices most likely have different physical pin layouts. The scramble feature permits a test developer to re-use test patterns for the different devices by re-programming only the scramble function. Re-use reduces the amount of time and effort to develop and debug DUT tests. There is a need, however, for additional features to enhance the mapping feature such as the ability to algorithmically manipulate a subset of the x-address lines or the y-address lines separate from another subset of the same x-address lines or y-address lines.

With specific reference to FIG. 3 of the drawings, there is shown a mid-level detail view of the address mapper 29 shown in FIG. 2 of the drawings in which XScramble 302, YScramble 304, and Z-Scramble 306 blocks each receive 16-bits of address from the Address X, Address Y, and Address Z ALU registers 24, respectively. Accordingly, the forty-eight(48) address lines shown as reference numeral 27 in FIG. 2 are broken into three(3) groupings of 16-bit logical address lines in FIG. 3. The XScramble, YScramble, and Z-Scramble blocks 302, 304, 306 also receive information that originates in APG registers 308 through 328. The XScramble, YScramble, and ZScramble blocks 302, 304, 306 perform a mapping function and output three(3) sets of 16-bit actual address information 330 into programmable latency logic 332. The programmable latency logic 332 performs the function that permits the test developer to program a latency as an integer multiple of instruction cycle times between address information presented to the DUT and DUT transmit or response data information. The programmable latency logic 332 outputs forty-eight(48) cycle latency adjusted address lines 334 which are input into the Auxiliary RAM 31.

With specific reference to FIG. 4 of the drawings, there is shown a block diagram of the programmable latency block 332 according to the teachings of the present invention. The programmable latency block 332 is part of the address mapper 29 shown in FIG. 2 and receives the outputs of the XScramble, YScramble, and ZScramble blocks 302, 304, 306 that comprise mapped x-, y-, and z-address lines 330. These mapped address lines 330 are hereinafter termed the mapped address word 330. The programmable latency block 332 delays the presentation of the mapped address word 330 a programmed number of instruction cycles until it correlates with the data information in the current program instruction cycle. The delayed mapped address word 330 as processed through the programmable latency block 332 is a cycle latency adjusted address word 334. Correlation of data received from the DUT 14 to the mapped address word 330 is accomplished by storing a comparison data word in the error catch RAM 32 that represents the comparison between the values returned by the DUT 14 and the values that are expected. The comparison data word is stored at an address location in the error catch memory 32 that is correlated to the address in the DUT 14. As an example, a one value in the comparison word that is stored in the error catch RAM 32 represents that the value of the channel as received by the DUT 14 is equal to the expected value of the channel during the current instruction cycle. A zero value in the comparison word represents an error on the channel during the current program instruction cycle. The cycle latency adjusted address word 334 is used to access specific locations in the error catch RAM 32 for storage of the comparison word in order to correlate the comparison data to the relevant DUT address. A feature according to the teachings of the present invention permits the test developer to hold the address information latent for a desired number of cycles and then permits the test developer to specify the cycle when the address information that is being held latent is to be used to access the error catch RAM 32, thus correlating address information with data information a number of cycles later in the testing process.

The data information that is applied to the DUT 14 often comes from either the “dh” and “dl” data registers or from the auxiliary RAM 31. The test developer holds the mapped address word 330 latent through use of a mnemonic that is optionally included as part of a program instruction word in test pattern code. In the disclosed embodiment, the test developer includes the “logAddr” mnemonic on a line of program code. The “logAddr” instruction sets a logAddr bit 602 in the 208-bit program instruction word for the instruction cycle in which it is included. The logAddr bit 602 is part of the various control values and instructions 42 shown in FIG. 2 of the drawings. The logAddr bit 602 is connected to a latency memory 604, a FIFO memory in the example, and triggers the latency memory 604 to store the forty-eight(48) bit mapped address word 330 in a first available write location. Each subsequent instruction that includes the logAddr bit 602 causes the latency memory 604 to store the mapped address word 330 in a next available write location. In this way, multiple words of mapped address words 330 may be temporarily stored for later use.

An “ecrlog” and “bmsel” mnemonics operate similarly to the “logAddr” mnemonic in that they may be included as part of a program instruction. Inclusion of the “ecrlog” mnemonic resets an ecrStore_N bit 606 in the 208-bit program instruction word for the instruction cycle in which it is included. Inclusion of the “bmsel” mnemonic sets a bmsel bit 614 in the 208-bit program instruction word for the instruction cycle in which it is included. The ecrStore_N bit 606 and the bmsel bit 614 are one of the various control values and instructions 42 shown in FIG. 2 of the drawings and are connected to the programmable latency block 332 in the address mapper 29. In this particular embodiment, the ecrStore_N bit 606 operates as a negative-true bit and the bmsel bit 614 operates as a positive-true bit. Accordingly, when the ecrStore_N bit 606 is reset, it is true and when the bmsel bit 614 is set it is true. A true ecrStore_N bit 606 or a true bmsel bit 614 sets an unload bit 616. When the unload bit 616 is true, it triggers the latency memory 604 to present the first available read location to a latency multiplexer 608. The latency multiplexer 608 comprises a forty-eight(48) channel 2×1 multiplexer that selects between a latency mode and a no-latency mode. A forty-eight(48) bit next available read location value in the latency memory 604 and the forty-eight(48) bit mapped address word 330 are presented as inputs to the latency multiplexer 608. The true unload bit 616 is also input into an AND gate 610 together with an “empty flag” 612 from the latency memory 604. A false “empty flag” 612 indicates that the latency memory 604 contains additional available read locations. As long as the latency memory 604 has available read locations, the unload bit 616 also controls the latency multiplexer 608 to select the input from the latency memory 604 for presentation at the output of the latency block 332 as the cycle latency adjusted address lines 334. A true “empty flag” 612 causes the latency multiplexer 608 to select the mapped address word 330 as the cycle latency adjusted address lines 334 of the programmable latency block 332 regardless of the value of the unload bit 616.

With specific reference to FIG. 6 of the drawings, there is shown a block diagram of the components of the data multiplexer 35 in which data from the dl and dh ALU registers 24 provide 32-bits of data 28 to a first data multiplexer 802. The first data multiplexer 802 comprises thirty-two(32) independently selectable 2×1 multiplexers. Each of the 32-bits of data 28 pass through thirty-two(32) inverters 804 to provide an inverted second input for each of the 32-bits of data for respective ones of the 2×1 multiplexers in the first data multiplexer 802. A thirty-two(32) bit wide output from the auxiliary RAM 31 is used to select each one of the 32-bits or their inverse in the first data multiplexer 802. The thirty-two(32) bit output of the first data multiplexer 802 is fed into a second data multiplexer 806. Also fed into the second data multiplexer 806 is the output from the auxiliary RAM 31. The second data multiplexer 806 comprises a thirty-two(32) channel 2×1 multiplexer. A configuration register that is typically programmed prior to test pattern execution has two bits dedicated to it for an auxiliary memory mode select 810. A mode select AND gate 808 accepts the auxiliary memory mode select bits 810 to generate a signal that selects either the output of the first data multiplexer 802 or the auxiliary RAM 31 to be presented as the output of the second data multiplexer 806. A third data multiplexer 812 accepts the thirty-two(32) bit output of the second data multiplexer 806 and the two(2) 16-bit dh and dl registers from the ALUs 24. The third data multiplexer 812 is a thirty-two(32) channel 2×1 multiplexer. The bmsel bit 614 selects either the thirty-two(32) bits of dh and dl register outputs from the ALUs 24 or the output of the second data multiplexer 806 for presentation as the output of the third data multiplexer 812.

With specific reference to FIG. 5 of the drawings, there is shown sample program code illustrating the use of the “logAddr” and “ecrlog” mnemonics in a programmable latency function. The example in FIG. 5 utilizes data from the dh and dl registers in the ALUs 24. Accordingly, data information flows through the third data multiplexer 812 and is selected by a false bmsel bit 614. For purposes of this example, the value of the mode select 810 is irrelevant. Line 1702 establishes a test pattern named “LATENCY”. Lines 2-3 704 initialize certain ALU registers and format the data lines, specifically the “dl” register in the ALUs 24, for a “no compare” meaning that the data lines are electrically tri-stated. Line 4 706 sets the ymatch register to a value of 6 and in the same instruction sets the logaddr bit 602 to store the current value of the mapped address word 330 in the first write location in the latency memory 604. Line 5 708 establishes an entry point named “LOOP1MAX”. Line 6 710 increments the y-address register by one(1) and jumps to the LOOP1MAX entry point if the y-address register is not yet equal to the ymatch register, 6 in the example. Line 6 710 also includes the “logAddr” mnemonic. For each increment of the y-address register up to and including the value of 6, therefore, the mapped address word 330 is stored in incrementing locations of the latency memory 604. Line 7 712 sets the dlmatch and ymatch registers to a value of 14. Line 8 714 increments the y-address and dl data registers by one(1), formats the data to compare with no load applied to the channels and includes both the “logAddr” and “ecrlog” mnemonics in the instruction word. The two mnemonics cause both the logAddr bit 602 and the ecrStore_N bit 606 to be set true simultaneously. Accordingly, the mapped address word 330 is stored in the next available write location in the latency memory 604 and the next read location in the latency memory 604 is presented to the latency multiplexer 608. Because the latency memory 604 is not empty, the next read location in the latency memory 604 is presented as the cycle latency adjusted address lines 334 at the output of the programmable latency block 332 and address mapper 29. The cycle latency adjusted address lines 334 access the specified location in the error catch RAM 32 at the same time that the dl register value is selected through the data multiplexer 35 and presented by the receive vector compare 40. The ecrStore_N bit 606 triggers the error catch RAM 32 to store the comparison data 61 at the location specified by the cycle latency adjusted address lines 334, 30. Line 9 716 establishes the LOOP2MAX entry point. Line 10 718 increments the y-address and dl-data registers by one(1). Line 10 718 also jumps to the LOOP2MAX entry point if the y-address register is not equal to the ymatch value, 14 in the example. The data is formatted for a compare with no load on the channels. Also included on line 10 718 are the “logaddr” and “ecrlog” mnemonics, which set the logAddr bit 602 and resets the ecrStore_N bit 606. Throughout the loop, therefore, the mapped address word 330 is loaded into the next available write location in the latency memory 604 and entries that are stored in the latency memory 604 are unloaded from the next available read locations and are presented as the cycle latency adjusted address word 334. Each cycle latency adjusted address word 334 at the output of the programmable latency block 332, therefore, is used to access the error catch RAM 32 with data that is seven(7) cycles latent from the current cycle. Line 11720 establishes LOOP3MAX as an entry point. Line 12 722 increments the dl data register by one(1) and formats the data channels for a compare with no load applied. Line 12 also jumps to the LOOP3MAX entry point if the dl data register does not equal the value in the dlmatch register, 14 in the example. Line 12 722 also includes the “ecrlog” mnemonic which causes the latency memory 604 to present the next available write location as the cycle latency adjusted address word 334 at the output of the programmable latency block 332. Line 12 722 repeats until the contents of the latency memory 604 are purged and the latency memory 604 is empty. Line 13 724 quits the LATENCY pattern.

In the example shown in FIG. 5, the dl register in the ALU 24 supplies the data information for comparison. The latency function can also be used, however, to access data information that is stored in the auxiliary memory 31 through use of the “bmsel” mnemonic. With specific reference to FIGS. 2 and 6 of the drawings, in a different use of the latency function, the auxiliary memory 31 is loaded with data information prior to execution of the test pattern. The “logAddr” mnemonic is used in the same way as shown in the example in FIG. 5. For the instruction cycle in which the data from the auxiliary memory 31 is to be used, the test developer includes the “bmsel” mnemonic. The bmsel bit 614 triggers the latency memory 604 to present the next available read location to the latency multiplexer 608 and also triggers the latency multiplexer 608 to select the address information from the latency memory 604 to be presented as the cycle latency adjusted address information 334 at the output of the address mapper 29. The cycle latency adjusted address information 334 is presented to the auxiliary RAM 31 to access the location specified by the cycle latency adjusted address information 334. The auxiliary RAM 31 then presents the accessed data to the data multiplexer 35. When using the auxiliary memory 31 for data comparison information, the mode select 810 is programmed such that the second data multiplexer 806 selects the auxiliary RAM output 33 to be presented at the output of the second data multiplexer 806. The bmsel bit 614 is used as a select control for the third data multiplexer 812 and selects the output of the second data multiplexer 806 which is the stored data 33 in the auxiliary RAM 31, as accessed by the cycle latency adjusted address information 334, to be presented at the output 38 of the data multiplexer 35. Optionally, the “ecrStore” mnemonic may also be used in conjunction with the “logAddr” and “bmsel” mnemonics in the same program instruction to not only provide comparison data from the auxiliary memory that is latent from the current ALU address register values, but to also store bit for bit results of the comparison in the error catch RAM 32 correlated to the appropriate cycle latency adjusted address information 334.

The present disclosure discusses features of the present invention, which are described by way of example. One of ordinary skill in the art appreciates that the present invention may be scaled without departing from the scope of the claims. The present disclosure is intended to be illustrative and not limitative of the present invention, the scope of the invention being defined solely by the appended claims. 

What is claimed is:
 1. An apparatus in a memory tester for correlating transmit and response_data information with address information for a device under test, said transmit and response data being_latent from said address information comprising: a first in first out latency memory having said address information available to it, a first signal synchronized with a first instruction cycle in said memory tester causing said latency memory to store said address information in a next available write location in said latency memory, and a second signal synchronized with a second instruction cycle in said tester causing said latency memory to retrieve said address information from a next available read location in said latency memory, and a tester memory that accepts said retrieved address information from said latency memory for accessing storage locations in said tester memory to store said transmit and response data information.
 2. An apparatus for correlating data information latent from address information in a memory tester as recited in claim 1 and further comprising a multiplexer to select between said retrieved address information and current address information.
 3. An apparatus for correlating data information latent from address information in a memory tester as recited in claim 1 wherein said tester memory stores comparison results in a location specified by said retrieved address information.
 4. An apparatus for correlating data information latent from address information in a memory tester as recited in claim 3 wherein said tester memory additionally provides comparison data from a location specified by said retrieved address information.
 5. An apparatus for correlating data information latent from address information in a memory tester as recited in claim 1 wherein said tester memory provides comparison data from a location specified by said retrieved address information.
 6. A method performed in a memory tester for correlating transmit and response data information with address information from which said data information is latent in a device under test comprising the steps of: storing said address information used to access a location in the device under test during a first instruction cycle, retrieving said stored address information during a second instruction cycle, accepting said transmit and response data information from said device under test during said second instruction cycle and using said retrieved address information stored in said first instruction cycle to access a location in a tester memory for storage of said transmit and response data information received during said second instruction cycle.
 7. A method as recited in claim 6, said second instruction cycle being one or more instruction cycles latent from said first instruction cycle.
 8. A method as recited in claim 6, the step of using said address information further comprising the steps of accessing said location in said memory for retrieval of comparison data and accessing said location in another memory for storage of comparison results.
 9. A method as recited in claim 6, the step of using said address information further comprising accessing said location in said memory for retrieval of said data information.
 10. A method as recited in claim 6 and further comprising the step of disabling said step of retrieving if said memory is empty.
 11. An apparatus for testing a memory device comprising: a sequencer for executing test program instructions, one or more of said instructions generating a write trigger and one or more of said instructions generating a read trigger, a first in first out latency memory receiving a write signal initiated by said write trigger that causes information to be written to a next available write location in said latency memory, said latency memory also receiving a read signal initiated by said read trigger that causes information to be read from a next available read location in said latency memory, a source of address information as data to said latency memory, a data source, and a tester memory receiving address information from said latency memory and for directing a location of storage in said tester memory of information from said data source.
 12. An apparatus as recited in claim 11 wherein said data source comprises a result of a comparison between received signals from a device under test and expected results from said device under test.
 13. An apparatus as recited in claim 12 wherein said expected results are generated an arithmetic logic unit.
 14. An apparatus as recited in claim 13 wherein said expected results are disposed in an auxiliary memory.
 15. An apparatus as recited in claim 14 wherein said digital information is address information and said auxiliary memory and said tester memory are addressed by said address information. 